Bone plates are devices for fixing bone, to facilitate repair of individual bones or joining of adjacent bones. Most commonly, bone plates are used to fix bone discontinuities, such as fractures or osteotomies, in individual bones. To fix such a bone discontinuity, a bone plate is attached to opposing sides of the discontinuity so that fragments of the affected bone can be held and/or grow together to repair the bone. However, in some cases, bone plates are used to join distinct bones at articular ends of the bones. For example, some articular injuries and/or diseases are treated effectively by bone fusion with a bone plate. Cartilage that separates articulating bones is removed, and a bone plate is attached. The plate spans a junction between the bones to fix the bones in close apposition so that the articular ends of the bones can fuse through bone growth.
The hand is a common site for performing bone fusion, for example, to treat osteoarthritis. Typically, four of the carpal bones are fused so that these bones can no longer move relative to one another. A bone plate for fixing carpal bones during and after bone fusion is disclosed in U.S. Pat. No. 6,179,839 to Weiss et al., issued Jan. 30, 2001 (the “Weiss patent”), which is incorporated herein by reference. The Weiss patent provides a substantially conical bone plate that is attached with bone screws at the junction of four carpal bones. The junction is modified prior to plate attachment to include a matching conical recess, by removing a portion of each carpal bone. The fusion plate is placed in the conical recess and attached to recessed bone surfaces to dispose the plate in a less obtrusive position below the anatomical surfaces of the carpal bones. The cone angle of the conical plate may help to direct the screws at oblique angles relative to the anatomical surfaces of the carpal bones and in a generally radial pattern from the plate. As a result, the bone screws pull the carpal bones together around the bone plate, as the screws attach the plate to the carpal bones.
The bone plate disclosed in the Weiss patent (“the Weiss plate”) has a variety of shortcomings. For example, the Weiss plate lacks an effective mechanism for holding the plate in contact with the recessed bone surfaces as the bone screws attach the plate to the carpal bones. Accordingly, the Weiss plate has a tendency to move (e.g., pivot and/or translate) out of full contact with the recessed bone surfaces when a load is applied unevenly to the plate, such as when the first bone screw attaches the plate to bone. As a result, the Weiss plate may be attached so that the plate is shifted from its desired target position. In addition, the Weiss plate lacks openings that allow optimal placement of a bone screw into each target carpal bone. Accordingly, a surgeon using the Weiss plate may be unable to secure one or more of the target carpal bones to the plate, because the plate has openings that are spaced too widely from one another. Furthermore, the Weiss plate lacks a mechanism to prevent bone screws from backing out of their inserted positions, for example, when inserted into bone of poor quality. Accordingly, some of the bone screws that are intended to hold the plate in position may back out of full engagement with bone, resulting in irritation of soft tissue that overlies the plate. The Weiss plate also may be configured to be placed more deeply into bone than is necessary, because of the conical shape of the Weiss plate.